WNT Signaling
Wnt proteins are characterized by a high number of conserved cysteine residues. Although Wnt proteins carry an N-terminal signal peptide and are secreted, they are relatively insoluble due to a particular protein modification, cysteine palmitoylation, which is essential for Wnt function (Willert et al., 2003). The porcupine gene, which displays homology to acyl-transferases, and its worm homolog mom-1 are believed to encode the enzyme that is responsible for Wnt palmitoylation (Zhai et al., 2004). Other genes that are conserved and are essential for Wnt secretion, named wntless (wls) and evenness interrupted (evi), respectively. These genes encode a seven-pass transmembrane protein that is conserved from worms (mom-3) to man (hWLS).
In the absence of Wls/evi, Wnts are retained inside the cell that produces them. The Wntless protein resides primarily in the Golgi apparatus, where it colocalizes and physically interacts with Wnts. The retromer, a multiprotein complex involved in intracellular trafficking and conserved from yeast to man, is also essential for Wnt secretion and for the generation of a Wnt gradient (Coudreuse et al., 2006). The retromer complex is involved in recycling a Wnt cargo receptor (such as Wntless) between the default secretory pathway and a compartment dedicated to Wnt secretion. Wnt is thought to act as a morphogen (that is, a long range signal whose activity is concentration dependent) (reviewed in Logan and Nusse, 2004). Morphogen action may occur when the palmitoyl moiety constrains movement away from membranes or lipid particles. Thus, Wnts may be tethered to intercellular transport vesicles or lipoprotein particles (Panakova et al., 2005). Alternatively, Wnts may be transported by cytonemes, which are long, thin filopodial processes. Additionally, extracellular heparan sulfate proteoglycans (HSPG) act in the transport or stabilization of Wnt proteins.
Receptors, agonists, and antagonists for Wnts bind Frizzled (Fz) proteins, which are seven-pass transmembrane receptors with an extracellular N-terminal cysteine-rich domain (CRD) (Bhanot et al., 1996). The Wnt-Fz interaction appears promiscuous, in that a single Wnt can bind multiple Frizzled proteins (e.g., Bhanot et al., 1996) and vice versa. In binding Wnt, Fzs cooperate with a single-pass transmembrane molecule of the LRP family known as LRP5 and -6 in vertebrates (Pinson et al., 2000; Tamai et al., 2000). The transport of LRP5/6 to the cell surface is dependent on a chaperone called Mesd in mice (Culi and Mann, 2003; Hsieh et al., 2003). And consistent with a role of the Mesd chaperone in the transport of LRP5/6 transport, mutations in Mesd resemble loss of LRP5/6. Although it has not been formally demonstrated that Wnt molecules form trimeric complexes with LRP5/6 and Frizzled, surface expression of both receptors is required to initiate the Wnt signal.
The secreted Dickkopf (Dick) proteins inhibit Wnt signaling by direct binding to LRP5/6 (Glinka et al., 1998). Through this interaction, Dkk1 crosslinks LRP6 to another class of transmembrane molecules, the Kremens (Mao et al., 2002), thus promoting the internalization and inactivation of LRP6. An unrelated secreted Wnt inhibitor, Wise, also acts by binding to LRP (Itasaki et al., 2003), as does the WISE family member SOST (Li et al., 2005; Semenov et al., 2005).
Canonical Wnt Signaling
Once bound by their cognate ligands, the Fz/LRPcoreceptor complex activates the canonical signaling pathway. Fz can physically interact with Dsh, a cytoplasmic protein that functions upstream of β-catenin and the kinase GSK-3. Wnt signaling controls phosphorylation of Dsh (reviewed in Wallingford and Habas, 2005). Recent studies have indicated that the coreceptor LRP5/6 interacts with Axin through five phosphorylated PPP(S/T)P repeats in the cytoplasmic tail of LRP (Davidson et al., 2005; Zeng et al., 2005). Wnts are thought to induce the phosphorylation of the cytoplasmic tail of LRP, thus regulating the docking of Axin. GSK3 phosphorylates the PPP(S/T) P motif, whereas caseine kinase I-γ (CK1γ) phosphorylates multiple motifs close to the GSK3 sites. CK1γ is unique within the CK1 family in that it is anchored in the membrane through C-terminal palmitoylation. Both kinases are essential for signal initiation.
Wnt Target Genes
Loss of components of the Wnt pathway can produce dramatic phenotypes that affect a wide variety of organs and tissues. A popular view equates Wnt signaling with maintenance or activation of stem cells (Reya and Clevers, 2005). It should be realized, however, that Wnt signals ultimately activate transcriptional programs and that there is no intrinsic restriction in the type of biological event that may be controlled by these programs.
Thus, Wnt signals can promote cell proliferation and tissue expansion but also control fate determination or terminal differentiation of postmitotic cells. Sometimes, these disparate events, proliferation and terminal differentiation, can be activated by Wnt in different cell types within the same structure, such as the hair follicle or the intestinal crypt (Reya and Clevers, 2005). Numerous Tcf target genes have been identified in diverse biological systems. These studies tend to focus on target genes involved in cancer, as exemplified by the wide interest in the Wnt target genes cMyc and Cyclin D1.
The Wnt pathway has distinct transcriptional outputs, which are determined by the developmental identity of the responding cell, rather than by the nature of the signal. In other words, the majority of Wnt target genes appear to be cell type specific. It is not clear whether “universal” Wnt/Tcf target genes exist. The best current candidates in vertebrates are Axin2/conductin (Jho et al., 2002) and SP5 (Weidinger et al., 2005). As noted (Logan and Nusse, 2004), Wnt signaling is autoregulated at many levels. The expression of a variety of positive and negative regulators of the pathway, such as Frizzleds, LRP and HSPG, Axin2, and TCF/Lef are all controlled by the β-catenin/TCF complex.
Patterning of the embryo and cell specification events are activated by a few evolutionarily conserved pathways, one of which is the Wnt/β-catenin pathway. These signaling proteins are used repeatedly during development and in diverse regions. The canonical Wnt pathway has been shown to regulate cell fate decisions, cell proliferation, and cell migration in the embryo. Canonical Wnt signaling is important for neural development, neural crest specification and differentiation, and cardiac development. The signals are transduced in a cell-context dependent manner to result in rapid changes in gene transcription. Reported evidence indicates that canonical Wnt signaling during narrow windows has differential effects during cardiac specification and heart development.
Congenital birth defects can arise with embryonic exposure to therapeutic drugs, high levels of normal plasma metabolites, or other environmental factors. Congenital cardiac defects arising from lithium (Li) exposure, a drug used for management of mood disorders, or from elevated plasma homocysteine (HCy) often involve tricuspid, pulmonary or aortic valve defects; a thickened heart wall; and/or defects in the outflow tract. Cardiac abnormalities are accompanied by neural tube defects and craniofacial anomalies that occur through unknown mechanisms. Previous studies failed to define a developmental window of primary susceptibility or any specific pathway(s) that may be targeted. In addition, chronic knockout approaches were unable to determine when early effects may arise. Thus what is needed is a defined developmental window of primary susceptibility as well as the identification of any specific pathways that may be targeted.